"The Brazos River is being pounded, and all of that water is coming down from the tributaries and creeks into the river," she said. "All the roads around us are flooded. We don't have any evacuation routes to tell people to take."

Brazoria County sits between the Gulf of Mexico and Houston, which remains the site of Harvey's most appalling devastation. Houston police have rescued more than 3,500 people from the floodwaters, Police Chief Art Acevedo said.

That number is almost certain to keep rising, as countless residents remain trapped in their deluged homes three days after Hurricane Harvey struck.

"Every passing hour, more boats are getting into the water," Acevedo said Tuesday. "This is a catastrophic event."

Latest developments

-- Before arriving Texas, Trump pledged his support. "To the people of Texas and Louisiana, we are 100% with you," Trump said Monday, adding that he believes Congress will act quickly to provide disaster-relief funding.

-- Several rain gauges southeast of downtown Houston report more than 48 inches of rain from the storm known as Harvey. That marks the most rain ever recorded in the contiguous United States from a tropical storm that made landfall. The previous record was held by Tropical Storm Amelia, which hit Texas in 1978.

-- Houston's George R. Brown Convention Center is hosting nearly 10,000 evacuees, about double the number of cots available. Those who couldn't get a cot were given pillows and blankets to sleep on the floor, Red Cross spokeswoman Betsy Robertson said. Houston Mayor Sylvester Turner said city officials were looking for more shelter space.

-- Houston officials will not ask for immigration status or documentation from anyone at any shelter, according to tweets in English and Spanish from the city's verified account.

More wrath to come

As Tropical Storm Harvey hovered over the Gulf of Mexico on Tuesday, it threatened to dump an additional 15 inches of rain on southwestern Louisiana and southeastern Texas, including the inundated Houston area.

And the water won't stop rising anytime soon. Swollen rivers in east Texas aren't expected to crest until later this week, and federal officials are already predicting the deadly storm will drive 30,000 people into shelters and spur 450,000 people to seek some sort of disaster assistance.

Harvey could bring more "catastrophic and life-threatening" flooding before making landfall again Wednesday morning near the Texas-Louisiana border, according to the CNN Weather Center.

People have turned to the walkie-talkie app Zello to report their dire circumstances. Among them were an elderly couple trapped on a roof and a family caught in the maelstrom with three children, including one in the throes of a seizure and another with autism.

Stuck in a wheelchair as the water rose

Confined to a wheelchair as the floodwater rose, Karen Preston watched as Harvey's flood took over her home.

Coast Guard rescuers on water scooters plucked her from the flood and to safety -- but without her wheelchair.

"I can't walk. I just had surgery on my knee," Preston said. "Right now, I'm in so much pain."

She found herself Tuesday among nearly 10,000 strangers taking shelter at Houston's convention center. Many have no idea whether their homes have been destroyed.

"How do you go from here?" Preston said, choking back tears. "Where do you start? Where?"

Entire Texas National Guard activated

Thousands of calls for help have gone out across Houston.

State, local and military rescue units have plucked thousands of stranded residents from the water and deluged homes.

"None of us (is) going to give up," Acevedo said.

The Pentagon is identifying resources, including trucks, aircraft and troops, that can be dispatched for hurricane relief if the request comes, defense officials said. Texas Gov. Greg Abbott activated the entire Texas National Guard, roughly 12,000 Guardsmen, he said Monday.

In Harris County, home to Houston, authorities asked stranded people to hang sheets or towels from their homes so rescuers could spot them more easily.

The scope of how many people are trapped in flooded homes remains unclear.

Rep. Sheila Jackson Lee said she believes the number of trapped residents across Houston could be "tens of thousands."

Volunteers come to help

Volunteers with boats jumped in to help authorities in search-and-rescue efforts.

FEMA Administrator Brock Long encouraged more citizens to volunteer, saying recovery efforts would require community involvement. The National Voluntary Organizations Active in Disaster website can direct people to religious and nongovernmental agencies that are helping people in at least 30 counties, he said.

But as water levels have risen, so has the desperation.

People started to panic, rushing rescue boats and even shooting at them if they didn't stop, one volunteer rescuer said.

Clyde Cain of the Cajun Navy, a Louisiana-based rescue force that gained fame during Hurricane Katrina, said in one instance, a boat broke down, and while the crew sought shelter in a delivery truck, people tried to steal the inoperable boat.